Acoustic filters for air ducts



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- parts, forming the acoustic filter;

nited States Patent ACOUSTIC FILTERS non .tra peers Dominic DEustachio, Port Allegany, Pa., assigner to Pittsburgh Corning Corporation, Pittsburgh, Pa., a corporation of Pennsylvania Application February 17, 1956, Serial No. 566,159

Claims. (Cl. lille-S0) This invention relates to acoustic filters or sound absorbers for air ducts. Ducts which convey air also transmit sound. This sound transmission often is undesirable. A commonly used method for absorbing this sound is to line the duct with sound absorbing materials. Typical attenuation produced by such linings are shown in Table l.

Minimum attenuation generally required is to 30 db and occasions arise where to 60 db of quieting is required. Thus, one can easily see that very long runs of conventionally lined ducts are needed to give adequate sound reduction. This is expensive. In addition, there are many situations where space is not available for a long duct run. An example is the common problem of preventing sound transmission through Ventilating ducts between two adjacent otlices. An object of the present invention is to provide an acoustic iilter which, when inserted in the air duct, allows free ilow of air through the duct but gives a noise reduction of 20 to 60 dbA in a total length of 2 to 10 ft. with a reasonable portion of this reduction in the low frequency range (50 C. P. S. to 500 C. P. S.).

In the accompanying drawings which'illustrate a preferred embodiment of the-invention,

Figure l is a diagrammatic plan view of an air blower, an air duct and an acoustic filter inserted in the air duct; Figure 2 is a plan view of a block of open-celled cellular .glass which has been cut intovtwo bodies along a sinuous line, the two bodies, when assembled with other Figure 3 is a perspective view of the the top platebeing broken away; and

Figure 4 is a horizontal longitudinal section taken along the line IV-IV of Figure 3.

Referring more particularly to the accompanying drawings, there is shown an air blower 2 connected to an air duct 4. An acoustic lilter 6 is interposed in the air duct and is connected to it by llexible couplings 8. In owing through the lter 6, air ilows through a sinuous passage 10 formed in part by two bodies 12 and 14 of open-celled cellular glass. Open-celled cellular glass from which the bodies 12 and 14 are formed may be made by the method disclosed in my Patent No. 2,596,659 granted May 13, 1952 or fby other suitable method. It should be particularly noted that the two bodies 12 and 14 are formed of open-celled cellular glass, The imacoustic tilter,

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portance of this can be understood when it is remembered that most rigid materials are set into Vibration by sound and transmit these'vibrations rather freely. This phenomenon is referred to in acoustic parlance as telegraphing of sound. Now it is not possible to make an effective short length acoustic filter, without elaborate vibration breaks, of any rigid material which telegraphs the sound. Though the reasons are not completely understood, open-celled cellular glass does not telegraph sound to any appreciable extent. Hence, the invention here described becomes economical and effective when open-celled cellular glass is used because the filter is only required to attenuate the air-borne noise.

Referring to Figure 2, the two bodies 12 and 14 of r open-celled cellular glass are formed by cutting a block 16 of the material along the sinuous line 18. Referring now more particularly to Figures 3 and 4, each of the blocks 12 and 14 of open-celled cellular glass is provided with recesses 20 which open into the passage 10. The body 12 comprises peaks 12a and valleys 12b and the body 14 comprises peaks 14a and valleys 14b. A body 12 and a body 14 are placed on and cemented to a bottom plate 22, the bodies 12 and 14 being in spaced apart relationship so as to provide the passage 1() between them. Hot asphalt, neoprene cement or any other suitable cement may be used for joining the bodies 12 and 14 to the plate 22. A top plate 24 is then 'bonded to the bodies 12 and 14. The plates 22 and 24 secure the sound absorbing bodies 12 and 14 in spaced apart relation to form the passage 10. In the embodiment shown, the plates 22 and 24 are made of closed-cell cellular glass but they may be made of any other suitable material since it is not essential that they have sound absorbing properties. They may, however, if desired, be made of open-celled cellular glass or other porous material. According to the invention, at least the major part of the walls of the sinuous passage 10 are made of open-celled cellular glass.

Referring now more particularly to Figure 4, the recesses 20 act as damped resonant cavities for absorbing sound. The frequency to which a recess is tuned, i. e., the frequency at which sound absorption is most ellicient, is dependent upon Various factors among which are'the depth of the recess, the'diameter of the 'mouth of the recess, the spacing between the recesses, the acoustic characteristics of the material in which the recesses are formed and the shape of the recesses, i. e., either tapered or straight.

ln my acoustic filters, some of the recesses 20 in the face between two successive valleys are tuned to different frequencies than other of the recesses insaid face.v Generally speaking, the recesses 20a which are formed in the thick portions of the bodies 12 and 14, that is, the portions adjacent thel peaks 12a and 14a, are deeper and have a larger mouth diameter' than the remaining recesses 2Gb. Thus, in the embodiment shown in Figure 4, there 'are four recesses 20a adjacenteach ofthe peaks 12a,

these recesses 20a each having a mouth diameter of e and being spaced on 1%" centers. All of the remaining recesses Ztlb have mouth diameters'of 1A" and are spaced from each other on 1/2 centers. The large diameter deep recesses 20a 'are best for low frequency sound absorption and the smaller diameter shallower recesses 20h are best for intermediate frequencies. In the embodiment shown, all of the recesses are tapered but, it desired, they may be made straight or some of them tapered and some of them straight. The height of the peaks 12a and 14a above the valleys 12b and 14b, this height being designated by reference letter h in Figure 4, is at least 3". ln the preferred embodiment, the peaks 14a of the body 14 and the peaks 12a of the body 12 lie approximately in a single longitudinal line L extending through the filter as shown in Figures 1 and 3. In any event, it is desirable that the transverse distance between the line connecting the peaks on the body 12 and the line connecting the peaks on the body 14 be not more than one-fifth of the height h of the peaks above the valleys.

In the embodiment shown, a single acoustic filter 6 is interposed in the air duct 4. However, if it is desired to increase the air handling capacity beyond that which can be obtained by a single filter, one can use a plurality of acoustic filters arranged in parallel to provide a plurality of passages arranged in parallel connected to the air duct.

The invention is not limited to the preferred embodiment but may be otherwise embodied or practiced within the scope of the following claims.

I claim:

1. An acoustic filter for an air duct, comprising a body l having a sinuous passage extending through it, at least the major part of the walls of said passage being made of open-celled cellular glass and having, in addition to the cells of the cellular glass, resonant cavities formed therein opening into said passage, said resonant cavities extending into said body of cellular glass for a substantial distance and terminating therein.

2. An acoustic filter for an air duct, comprising two spaced apart bodies made of open-celled cellular glass and forming between their adjacent faces at least the major part of the walls of a sinuous passage extending through said filter, each of said faces having, in addition to the cells of the cellular glass, resonant cavities formed therein opening into said passage, said resonant cavities extending into said body of cellular glass for a substantial distance and terminating therein.

3. An acoustic filter for an air duct, comprising two spaced apart bodies made of open-celled cellular glass and forming between their adjacent faces two walls of a sinuous passage extending through said filter, said two walls forming the major part of the walls of the sinuous passage, each of said adjacent faces comprising alternate Vpeaks and valleys, the peaks on one of said bodies being located opposite the valleys of the other body, each of said faces having, in addition to the cells of the cellular glass, resonant cavities formed therein opening into said passage, said resonant cavities extending into said body of cellular glass for a substantial distance and terminating therein.

4. An acoustic filter according to claim 3, wherein the peaks of the spaced apart bodies lie approximately in a single longitudinal line extending through said filter.

v5. An acoustic filter according to claim 3, wherein the transverse distance between the line connecting the peaks on one of said bodies `and the line connecting the peaks on the other of said bodies is not more than one-fifth of the height of said peaks above said valleys.

6. An acoustic filter according to claim 3, wherein some of said resonant cavities extend into said body of cellular glass for a distance of at least 3".

7. An acoustic filter for an air duct, comprising two spaced apart bodies made of open-celled cellular glass and forming between their adjacent faces two walls of a sinuous passage extending through said filter, said two walls forming the major part of the walls of the sinuous passage, each of said adjacent faces comprising alternate peaks and valleys, the peaks on one of said bodies being located opposite the valleys of the other body, each of said faces having, in addition to the cells of the cellular glass, resonant cavities formed therein opening into said passage, said resonant cavities extending into said body of cellular glass for a substantial distance and terminating therein, some of said resonant cavities in each face between two successive. valleys being tuned to different frequencies than other of said recesses in said face.

8. An acoustic filter for an air duct, comprising two spaced apart bodies made of open-celled cellular glass and forming between their adjacent faces two Walls of a sinuous passage extending through said filter, said two walls forming the major part of the walls of the sinuous passage, each of said adjacent faces comprising alternate peaks and valleys, the peaks on one of said bodies being located opposite the valleys of the other body, each of said faces having, in addition to the cells of the cellular glass, resonant cavities formed therein opening into said passage, said resonant cavities extending into said `body of cellular glass for a substantial distance and terminating therein, some of said resonant cavities in each face between two successive valleys being at least 3" deep and being deeper than other of said resonant cavities in said face.

9. An acoustic filter for an air duct, comprising a body having a sinuous passage extending through it, at least the major part of the walls of said passage being made of open-celled cellular glass and having, in addition to the cells of the cellular glass, cavities formed therein opening into said passage, said cavities extending into said body of cellular glass for a substantial distance, said distance and the cross sectional area of each of said cavities being selected so that the impedance of the combination of said cavities and opencelled cellular glass in which the cavities are made is substantially shifted toward negative reactance values and made in this manner to approach an optimum value forsound absorption in any desired frequency range.

10. An acoustic filter for an air duct, comprising a body having a sinuous passage extending through it, at least the major part of the walls of said, passage being made of open-celled cellular frangible material and having, in addition to the cells of the open-celled cellular frangible material, resonant cavities formed therein opening into said passage, said resonant cavities extending into said body of open-celled cellular frangible material for a substantial distance and terminating therein.

References Cited in the file of this patent UNITED STATES PATENTS 2,596,659 DEustachio May 13, 1952 2,703,627 DEustachio Mar. 8, 1955 2,759,554 Baruch Aug. 21, 1956 FOREIGN PATENTS I 8,529 Australia July 26, 1928 of 1927 Y 449,507 Great Britain June 29, 1936 

